The sudden decision by the company’s chief executive to take a medical leave for the third time in less than a decade raises anxieties about the leadership of the company he helped found more than three decades ago. It also puts the spotlight again on several senior executives who have been helping Mr. Jobs run the company, in particular Timothy D. Cook, the chief operating officer, who will take over day-to-day operations during Mr. Jobs’s leave.

Whether Mr. Jobs returns, as he said he hoped to, or does not, as some investors fear, most experts believe that despite having a strong executive team, he will be hard to replace.

“The company could not thrive if Steve didn’t have an extremely talented team around him,” said David B. Yoffie, a professor at Harvard Business School who has studied the technology industry for decades. “But you can’t replace Steve on some levels.”

Mr. Yoffie and others said Mr. Jobs’s creativity, obsession with a product’s design and function, and management style, as well as the force of his personality, were unusual, not only in Silicon Valley, but also in American business. They said that it would take several people with different skills to fill Mr. Jobs’s shoes.

“The person who can keep the trains running on time is a scarce commodity, but not as rare as someone who can do breakthrough innovation,” said Michael Useem, a professor at the Wharton School of the University of Pennsylvania and the director of its Center for Leadership and Change Management.

His performance during that time provides a heavy dose of reassurance for nervous investors. Mr. Cook, who joined Apple nearly 13 years ago and is otherwise responsible for the company’s worldwide sales and operations, steered the company successfully the last time around. He kept the development of products like the iPhone 4 and the iPad on track, increased Macintosh computer sales and improved Apple’s financial performance during an economic downturn.

While Apple shares dipped when Mr. Jobs announced his 2009 leave, they rallied strongly during Mr. Cook’s tenure. Before joining Apple, Mr. Cook was vice president of corporate materials for Compaq, which was bought by Hewlett-Packard.

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Stepping In Timothy Cook, left, with Steven Jobs in July, kept the development of products like the iPad on track and improved Apple’s financial performance when Mr. Jobs took a medical leave in 2009.Credit
Kimberly White/Reuters

While Mr. Jobs has remained intimately involved in the company despite his health, Mr. Cook’s responsibilities, and his position as apparent heir to Mr. Jobs, have strengthened.

“I was with Tim Cook last week in New York and I walked away from that thinking, ‘This guy is more in charge and more in control of Apple than I think people understand,’ ” said Tim Bajarin, an analyst with Creative Strategies who has followed Apple for nearly three decades. “He clearly is the guy that if Apple needed additional leadership at the top, could actually carry it.”

Analysts said that like Mr. Jobs, Mr. Cook is obsessed with details and involved in minute elements of the business. “Tim is obsessive about operational detail, and Steve is obsessive about product detail,” said Gene Munster, an analyst with Piper Jaffray.

A handful of other executives, whose roles are complementary to that of Mr. Cook’s, are also expected to see their profiles rise in Mr. Jobs’s absence. They include Jonathan Ive, a London-born designer who is Apple’s senior vice president for industrial design and close to Mr. Jobs. “He’s arguably the most important person there outside of Steve,” said Shaw Wu, an analyst at Kaufman Brothers. “He’s responsible for the look and feel of the products, the way they interact with users.”

Philip W. Schiller, the company’s marketing chief, is also expected to play a vital role. During Mr. Jobs’s last leave, it was Mr. Schiller who took over as Apple’s chief showman, taking the stage at events typically headlined by Mr. Jobs. Among other products, he unveiled the iPhone 3GS as well as an updated line of MacBook Pro laptops.

And Scott Forstall, senior vice president for iPhone software, is also believed to have an increasingly influential role as software becomes the distinguishing factor on phones and tablets. He has been behind Apple’s push to unify the software that powers its mobile devices and its Macintosh computers.

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Jonathan Ive Senior vice president of industrial design, he is responsible for the look and feel of products.Credit
Jim Wilson/The New York Times

Despite his recurring health problems, Mr. Jobs has remained involved in decisions big and small. Last year, for instance, he took center stage in the company’s response to a public relations issue over problems with the iPhone 4’s antenna and participated in discussions with Facebook over the release of Ping, Apple’s social networking service inside iTunes.

One area where Mr. Jobs’s influence will be hard to replace is at the negotiating table. Mr. Jobs has sought to sway many through the strength of his personality, for instance, playing a direct role in persuading media companies to make their content available on Apple’s products. That role is increasingly important, as Apple seeks to become an even bigger power in media distribution. Mr. Jobs has some recent victories on that front; for instance, when the Beatles agreed to sell their music on iTunes. He was also able to persuade Disney and the News Corporation to make some television shows available for 99 cents through the Apple TV device.

“When Steve talks, people listen,” Mr. Yoffie said. “It would be hard for anyone in the industry to have a comparable level of influence.”

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Scott Forstall Senior vice president of iPhone software, he is working to unify the company’s software.Credit
Ryan Anson/Bloomberg News

No one expects Apple to suffer in the short term, as the company has a long product cycle. But some raise questions as to what will happen over the long term if Mr. Jobs does not return.

“The problem here isn’t the operations of Apple and their ability to execute and keep doing what they’ve been doing,” Mr. Munster said. “As far as what they’ve got in place, no doubt they can deliver, but as far as inspiring products you haven’t thought up yet, that’s what you’re going to lose.”

“The problem, really at the core,” he said, “is that Steve Jobs’s inspiration is irreplaceable.”

A version of this article appears in print on January 18, 2011, on Page B1 of the New York edition with the headline: Despite Anxiety Over Leave at Apple, a Deep Bench of Leadership. Order Reprints|Today's Paper|Subscribe